A Simple Yet Effective Model Organism: Schmidtea mediterranea

Schmidtea mediterranea: some generalities

Schmidtea mediterranea is a very unique member of the Platyhelminthes phylum and can be found in the Lophotrochozoan clad. It is a proteostome and has also been classified as triploblastic (they form three germ layers during gastrulation), unsegemented, and bilateral. They possess a blind gut, lack an anus, and show cephalization. They lack both circulatory systems and respiratory systems and must therefore obtain oxygen for their cells via diffusion. For more general information on flatworms feel free to visit the Wikipedia page.

Schmidtea mediterranea has recently been chosen as a model organism for investigating stem cells, regeneration, germ cell specification, and sexual development. Why is it a good model for these studies?

For starters, this organism is relatively easy to culture in laboratory settings, it has developmental plasticity and two modes or reproduction (asexual and sexual), and it has a comparatively small, diploid genome. Their ability to regenerate allowed experimenters to create clonal lines of both sexes from single animals. They reproduce sexually in water temperatures below 200C and reproduce asexually in higher water temperatures allowing for control of reproduction.

One lead researchers studying this planarian is Dr. Alejandro Sánchez Alvarado and it is encouraged to visit his research website full of information and resources on this organism. In addition, one can view some of his lectures on youtube, below are three sequential videos.

Video 1

Video 2

Video 3

Regeneration:

Schmidtea mediterranea can regenerate entire, fully developed adults from small tissue fragments, making them an ideal model organism for the study of body axis polarization and patterning. Scientists have found several genes and pathways involved in reestablishing and maintaining the antero-posterior and dorso-ventral polarity, including Smed-βcatenin-1, Wnt, and Hedgehog/Patched (antero-posterior axis); and BMP/ADMP and Noggin-like genes (dorso-ventral axis). Despite the enormous potential of these animals for medical research and regenerative medicine and the current research being conducted on planarian regeneration, all of the mechanisms and molecules involved in this phenomenal process are still unknown. For more information on regeneration in Schmidtea mediterranea, click the following link.

Like other basal and higher metazoans, Schmidtea mediterranea specifies it germ cells epigenetically instead of through maternal determinants. The exact pathway and identity of such epigenetic determination; however, is still being explored for this model organism but one protein in particular Smed-nanos has caught the attention of many developmental biologists. It is a homologue Nanos which has been found in many different organisms such as the fruit fly. In addition, Smed-nanos appears to be an essential signal molecule to the specification, regeneration, and maintenance of germ cells in Schmidtea mediterranea. For more in germ cell specification and Smed-nanos visit the following link.

Genes and Development Issue 18 2010 cover photo: In situ hybridization of Smed-hnRNPA2 mRNAs which are found in the spermatogonial cells and early spermatocytes in the dorsolaterally distributed testes lobules (germ cells). The rest of the organism was couterstained with DAPI blue to show the nuclei.

Sexual Development:

Schmidtea mediterranea is a well-known fresh water planarian known for its unique reproductive modes. The planarian is known to partake in two different types of reproduction, sexual and asexual; these distinct reproductive paths are useful in studying sexual reproduction. The sexual strains are cross-fertilizing hermaphrodites that develop post embryonically. The asexual strain on the other hand, reproduces through transverse fission and fails to develop reproductive organs. To learn more about the reproductive system of the planarian click on the following link.

Chong, T. et al. 2011 Fig. 1: Reproductive system of the Schmidtea mediterranea. (A) Reproductive system of the sexual strain. The right picture shows an enlarged view of the copulatory apparatus. (B) Shows a view of different germ cell types in the testes.

Stem cells of Schmidtea mediterranea

Schmidtea mediterranea is quickly becoming a model organism for its stem cells, which are called neoblasts. These stem cells confer the ability of S. mediterranea to regenerate body parts throughout their lifetimes, as well as maintain tissues. Located throughout the worm, neoblasts are mitotically active and are the only constantly proliferating cells in the worm. Irradiation of neoblasts causes arrest of neoblast activity and thus halts regeneration. The fate of the cells produced by neoblasts depends on the genes active both spatially and temporally in the worm. Identifying the genes active in neoblasts and neoblast descendants could provide insight into general stem cell biology of other animals, as the molecular procedures of stem cell regulation is likely conserved over evolution of metazoans. For more information on S. mediterranea stem cells check out the following link.